Bottom Line:
Numerous examples exist of how disrupting the actions of physiological regulators of blood cell development yields hematologic malignancies.These pathophysiologies include myelodysplastic syndrome, acute myeloid leukemia and an immunodeficiency syndrome with complex phenotypes including leukemia.As GATA-2 has a pivotal role in the etiology of human cancer, it is instructive to consider mechanisms underlying normal GATA factor function/regulation and how dissecting such mechanisms may reveal unique opportunities for thwarting GATA-2-dependent processes in a therapeutic context.

Affiliation: Wisconsin Institutes for Medical Research, Paul Carbone Cancer Center, Department of Cell and Regenerative Biology, University of Wisconsin School of Medicine and Public Health, Madison, WI 53705, USA. ehbresni@wisc.edu

ABSTRACTNumerous examples exist of how disrupting the actions of physiological regulators of blood cell development yields hematologic malignancies. The master regulator of hematopoietic stem/progenitor cells GATA-2 was cloned almost 20 years ago, and elegant genetic analyses demonstrated its essential function to promote hematopoiesis. While certain GATA-2 target genes are implicated in leukemogenesis, only recently have definitive insights emerged linking GATA-2 to human hematologic pathophysiologies. These pathophysiologies include myelodysplastic syndrome, acute myeloid leukemia and an immunodeficiency syndrome with complex phenotypes including leukemia. As GATA-2 has a pivotal role in the etiology of human cancer, it is instructive to consider mechanisms underlying normal GATA factor function/regulation and how dissecting such mechanisms may reveal unique opportunities for thwarting GATA-2-dependent processes in a therapeutic context. This article highlights GATA factor mechanistic principles, with a heavy emphasis on GATA-1 and GATA-2 functions in the hematopoietic system, and new links between GATA-2 dysregulation and human pathophysiologies.

gks281-F1: GATA factor mechanistic principles. The models depict mechanistic principles derived from studies of GATA-1 and GATA-2. While the fundamental nature of these principles is likely to be shared by other GATA factors, additional GATA factor-specific mechanistic permutations are expected. Principle 1: GATA factors occupy a very small percent of the WGATAA motifs in a genome (<1%), suggesting that crucial mechanisms exist that control the discrimination among these highly abundant motifs. However, such mechanisms are not firmly established. The model depicts the occlusion of select GATA motifs, thus creating an obligate requirement for chromatin remodeling/modification reactions to increase access of the WGATAA residues required for GATA factor binding and/or to selectively occlude the vast majority of sites. At certain sites, FOG-1 (56,57) and GATA-1 acetylation (95) enhance chromatin access. Presumably, a host of regulatory factors mediate the essential process of establishing/maintaining accessible and occluded sites. Principle 2: GATA factors activate and repress target genes via multiple mechanisms, including with or without FOG-1 (36). Presumably, this mechanistic diversity reflects the specific chromatin architecture at a genetic locus, the subnuclear environment in which the locus resides and the regulatory mileau characteristic of the specific environment. Principle 3: GATA-1 and GATA-2 commonly co-localize with Scl/TAL1, another master regulator of hematopoiesis (96), at chromatin sites. The model illustrates GATA factor and Scl/TAL1 occupancy of a composite element consisting of an E-box and a WGATAA motif, which was originally described by Wadman et al. (76). Similar to the description above, only a very small percentage of composite elements are occupied by GATA factors in cells (53,58). As co-localization does not require the E-box (72), there is much to be learned about the biochemical nature of the GATA factor and Scl/TAL1 interaction. However, the co-localization measured by ChIP often correlates with transcriptional activity (54,58,72). Principle 4: GATA switches are defined as a molecular transition in which one GATA factor replaces another from a chromatin site, which is often associated with an altered transcriptional output. The GATA switch depicted reflects that occurring at the Gata2 locus during erythropoiesis, in which GATA-1 displaces GATA-2 from chromatin, which rapidly instigates repression (87). Context-dependent GATA switches may either activate or repress transcription and, in certain cases, may sustain the original transcriptional output (36).

Mentions:
Despite approximately 7 million GATA motifs in the human genome, all capable of forming high-affinity complexes with GATA factors and naked DNA in vitro, GATA-1 and GATA-2 occupy only 0.1–1% of these motifs in erythroblasts, based on chromatin immunoprecipitation coupled with massively parallel sequencing and real-time PCR validation (14,53,54). While the molecular determinants for this exquisite discrimination are not fully understood (55), FOG-1 facilitates GATA-1 occupancy at a subset of chromatin sites (56,57). Genome-wide analysis of cis-elements residing at endogenous GATA-1 and GATA-2 occupancy sites led to refinement of the GATA consensus from WGATAR to WGATAA. However, the percent of total WGATAA motifs occupied remains very low. Beyond GATA motif sequence composition, the most rudimentary determinant of chromatin occupancy, diagnostic patterns of histone posttranslational modifications demarcate occupied versus unoccupied sites, both containing conserved GATA motifs (53,58–60) (Figure 1, Principle 1). In principle, the unique epigenetic signature of occupied sites may represent primed chromatin structures recognized by GATA-1 as a pivotal determinant of site selection. Alternatively, the signature may arise as a consequence of GATA-1 chromatin occupancy, followed by recruitment of GATA-1 coregulators that modify chromatin surrounding the occupancy site.

gks281-F1: GATA factor mechanistic principles. The models depict mechanistic principles derived from studies of GATA-1 and GATA-2. While the fundamental nature of these principles is likely to be shared by other GATA factors, additional GATA factor-specific mechanistic permutations are expected. Principle 1: GATA factors occupy a very small percent of the WGATAA motifs in a genome (<1%), suggesting that crucial mechanisms exist that control the discrimination among these highly abundant motifs. However, such mechanisms are not firmly established. The model depicts the occlusion of select GATA motifs, thus creating an obligate requirement for chromatin remodeling/modification reactions to increase access of the WGATAA residues required for GATA factor binding and/or to selectively occlude the vast majority of sites. At certain sites, FOG-1 (56,57) and GATA-1 acetylation (95) enhance chromatin access. Presumably, a host of regulatory factors mediate the essential process of establishing/maintaining accessible and occluded sites. Principle 2: GATA factors activate and repress target genes via multiple mechanisms, including with or without FOG-1 (36). Presumably, this mechanistic diversity reflects the specific chromatin architecture at a genetic locus, the subnuclear environment in which the locus resides and the regulatory mileau characteristic of the specific environment. Principle 3: GATA-1 and GATA-2 commonly co-localize with Scl/TAL1, another master regulator of hematopoiesis (96), at chromatin sites. The model illustrates GATA factor and Scl/TAL1 occupancy of a composite element consisting of an E-box and a WGATAA motif, which was originally described by Wadman et al. (76). Similar to the description above, only a very small percentage of composite elements are occupied by GATA factors in cells (53,58). As co-localization does not require the E-box (72), there is much to be learned about the biochemical nature of the GATA factor and Scl/TAL1 interaction. However, the co-localization measured by ChIP often correlates with transcriptional activity (54,58,72). Principle 4: GATA switches are defined as a molecular transition in which one GATA factor replaces another from a chromatin site, which is often associated with an altered transcriptional output. The GATA switch depicted reflects that occurring at the Gata2 locus during erythropoiesis, in which GATA-1 displaces GATA-2 from chromatin, which rapidly instigates repression (87). Context-dependent GATA switches may either activate or repress transcription and, in certain cases, may sustain the original transcriptional output (36).

Mentions:
Despite approximately 7 million GATA motifs in the human genome, all capable of forming high-affinity complexes with GATA factors and naked DNA in vitro, GATA-1 and GATA-2 occupy only 0.1–1% of these motifs in erythroblasts, based on chromatin immunoprecipitation coupled with massively parallel sequencing and real-time PCR validation (14,53,54). While the molecular determinants for this exquisite discrimination are not fully understood (55), FOG-1 facilitates GATA-1 occupancy at a subset of chromatin sites (56,57). Genome-wide analysis of cis-elements residing at endogenous GATA-1 and GATA-2 occupancy sites led to refinement of the GATA consensus from WGATAR to WGATAA. However, the percent of total WGATAA motifs occupied remains very low. Beyond GATA motif sequence composition, the most rudimentary determinant of chromatin occupancy, diagnostic patterns of histone posttranslational modifications demarcate occupied versus unoccupied sites, both containing conserved GATA motifs (53,58–60) (Figure 1, Principle 1). In principle, the unique epigenetic signature of occupied sites may represent primed chromatin structures recognized by GATA-1 as a pivotal determinant of site selection. Alternatively, the signature may arise as a consequence of GATA-1 chromatin occupancy, followed by recruitment of GATA-1 coregulators that modify chromatin surrounding the occupancy site.

Bottom Line:
Numerous examples exist of how disrupting the actions of physiological regulators of blood cell development yields hematologic malignancies.These pathophysiologies include myelodysplastic syndrome, acute myeloid leukemia and an immunodeficiency syndrome with complex phenotypes including leukemia.As GATA-2 has a pivotal role in the etiology of human cancer, it is instructive to consider mechanisms underlying normal GATA factor function/regulation and how dissecting such mechanisms may reveal unique opportunities for thwarting GATA-2-dependent processes in a therapeutic context.

Affiliation:
Wisconsin Institutes for Medical Research, Paul Carbone Cancer Center, Department of Cell and Regenerative Biology, University of Wisconsin School of Medicine and Public Health, Madison, WI 53705, USA. ehbresni@wisc.edu

ABSTRACTNumerous examples exist of how disrupting the actions of physiological regulators of blood cell development yields hematologic malignancies. The master regulator of hematopoietic stem/progenitor cells GATA-2 was cloned almost 20 years ago, and elegant genetic analyses demonstrated its essential function to promote hematopoiesis. While certain GATA-2 target genes are implicated in leukemogenesis, only recently have definitive insights emerged linking GATA-2 to human hematologic pathophysiologies. These pathophysiologies include myelodysplastic syndrome, acute myeloid leukemia and an immunodeficiency syndrome with complex phenotypes including leukemia. As GATA-2 has a pivotal role in the etiology of human cancer, it is instructive to consider mechanisms underlying normal GATA factor function/regulation and how dissecting such mechanisms may reveal unique opportunities for thwarting GATA-2-dependent processes in a therapeutic context. This article highlights GATA factor mechanistic principles, with a heavy emphasis on GATA-1 and GATA-2 functions in the hematopoietic system, and new links between GATA-2 dysregulation and human pathophysiologies.